1. Field of the Invention
The present invention relates to a driving circuit for a vacuum fluorescent display.
2. Description of the Related Art
A vacuum fluorescent display (hereinafter, referred to as “VFD”) is a display device of a self-illuminating type for displaying a desired pattern by causing a direct-heating type cathode called a filament to emit thermoelectrons by causing it to generate heat by applying a voltage thereto in a vacuum chamber and by causing the thermoelectrons to collide against fluorescent material on an anode (segment) electrode and causing them to illuminate, by accelerating the thermoelectrons using a grid electrode. VFDs have excellent features in terms of visibility, multi-coloring, a low operating voltage, reliability (environmental resistance) etc. and are used in various applications and fields such as cars, home appliances and consumer products.
For a VFD, as one scheme for applying a voltage to its filament, pulse-driving scheme has been proposed. Pulse-driving scheme is a scheme in which a pulse voltage (hereinafter, referred to as “filament pulse voltage”) generated by chopping a DC voltage relatively high compared to the ordinary rated voltage of the filament is applied to the filament, and has features that illuminating state having a small intensity gradient is obtained, etc.
Here, conventional VFD driving circuits intended for driving the VFD are equipped with an arrangement to adjust the VFD brightness so that the VFD can display at a proper brightness according to the surrounding environmental conditions (e.g., ambient illuminance). For example, this arrangement includes methods called grid dimming in which the duty ratio of the voltage applied to the grid electrode (hereinafter referred to as “grid voltage”) is adjusted and anode dimming in which the duty ratio of the voltage applied to the segment (anode) electrode (hereinafter referred to as “segment voltage”) is adjusted. Grid dimming leads to an inconstant amount of thermoelectrons between the filament and the grid as a result of variations in grid voltage pulse width, allegedly degrading the VFD display integrity. For this reason, anode dimming is drawing more attention lately than grid dimming.
Grid/anode dimming is carried out, for instance, based on a comparison table between dimmer adjustment data and dimmer values as shown in FIG. 7A. It is to be noted that dimmer adjustment data—data brought in correspondence with values that can be set as duty ratio of grid or segment voltage—is specified when grid/anode dimming is externally conducted on a VFD driving circuit. Dimmer adjustment data may also be binary data having the number of bits appropriate to the grid/anode dimming resolution as in the case of 10-bit binary data (DM0 to DM9) shown in FIG. 7A with DM0 being the LSB (Least Significant Bit). Meanwhile, dimmer value—a value that can be set as a duty ratio of grid or segment voltage—can be defined, by using a pulse width TW and a pulse period T indicated in the waveform diagram of FIG. 7B, as “pulse width TW/pulse period T.”
Conventional VFD driving circuits have adopted one of the following to implement grid/anode dimming:
Embodiment A: Embodiment for performing grid dimming only (e.g., refer to “OKI Electronic Devices MSC12056 Datasheet (J2C0018-27-Y3)”, [online] Prepared January 1998, OKI Electric Industry, [Searched Mar. 27, 2003],
Embodiment B: Embodiment for performing anode dimming only (e.g., refer to “OKI Electronic Devices ML9213 Datasheet (FJDL9213-01)”, [online] Prepared September 2000, OKI Electric Industry, [Searched Mar. 27, 2003],
Embodiment C: Embodiment for performing both grid and anode dimming simultaneously (e.g., refer to “OKI Electronic Devices MSC1205-01 Datasheet (FJDL1215-03)”, [online] Prepared September 2000, OKI Electric Industry, [Searched Mar. 27, 2003],
Here, a phenomenon called “ghost failure” will be described with reference to FIGS. 8A to 8C as an example of display operation of a VFD having a two-digit seven segment display as its display pattern.
As shown in FIG. 8A, the digit corresponding to a grid voltage G1 is scanned (a grid electrode G1 is driven) at period 1T, and concurrently a segment electrode Sm is driven, lighting up a segment Sm (1) shown in FIG. 8B.
Next, the digit corresponding to a grid electrode G2 is scanned (the grid electrode G2 is driven) at period 2T, and concurrently the segment electrode Sm is driven, lighting up a segment Sm (2) shown in FIG. 8B. Here, the grid voltage applied to the grid electrode G1 is supposed to essentially drop to a level that does not drive the grid electrode G1 before the segment Sm (2) lights up, thus causing the segment Sm (1) that was lit at period 1T to go out.
As shown in a dashed line area P in FIG. 8A, however, the waveform of the grid voltage applied to the grid electrode G1 becomes dull due, for example, to resistive and capacitive components in the wiring between the output terminal of the VFD driving circuit and the VFD grid electrode G1. This results in a period during which both the segments Sm (1) and Sm (2) are lit as shown in FIG. 8B.
It is to be noted that such a phenomenon is generally called “ghost failure” and constitutes one of the factors degrading the VFD display integrity. To eliminate such “ghost failure”, VFD circuit must adjust the duty ratio of the grid voltage to an appropriate value (grid dimming), taking into consideration the effect of dulling of the waveform of the grid voltage applied to the grid electrode.
Meanwhile, “ghost failure” also takes place as shown in FIG. 8C in a dotted line area Q in FIG. 8A. In this case, the segment voltage applied to the segment electrode Sm is supposed to essentially drop to a level that does not drive the segment electrode Sm at period 4T before a segment Sn (2) shown in FIG. 8C lights up, thus causing the segment Sm (2) shown in FIG. 8C that was lit at period 3T to go out.
Because of the same cause as the dulling of the grid voltage waveform described earlier, however, the segment voltage applied to the segment electrode Sm dulls, resulting in a period during which both the segments Sm (2) and Sn (2) are lit as shown in FIG. 8C. In this case, the VFD circuit must adjust the duty ratio of the segment voltage to an appropriate value (anode dimming), taking S into consideration the effect of dulling of the waveform of the segment voltage applied to the segment electrode.
The above is the description of the phenomenon called “ghost failure.” Incidentally, conventional VFD driving circuits are only capable of either grid or anode dimming in the case of the embodiment A or B, thus making it impossible to eliminate the “ghost failure.”
On the other hand, the embodiment C of conventional VFD driving circuits performs both grid and anode dimming concurrently to eliminate the “ghost failure.” However, grid dimming is performed simultaneously with anode dimming at a time when anode dimming alone is enough (e.g., the dotted line area Q in FIG. 8A). This leads to an unstable amount of thermoelectrons between the filament and grid electrodes, degrading the VFD display integrity.